Following a traumatic surgery such as a total hip replacement or total knee replacement, there is always inevitably a loss of muscle mass (known as atrophy) and overall muscle strength. This often presents functionally as difficulty with lifting a leg, standing up from a seated position, maintaining a standing position, walking, and going up and down stairs. When a person undergoes joint replacement surgery, they are educated on these potential side effects as well as what to expect in terms of post-operative rehabilitation. The rehab program can consist of techniques for controlling pain, reducing swelling, increasing flexibility, and increasing strength. Today, we will focus on the strengthening aspect of post-operative recovery, specifically exploring the difference between concentric and eccentric exercise.

First, it is important to understand the definitions of both concentric exercise and eccentric exercise. According to the National Institutes of Health, concentric muscle contraction occurs when “muscle tension rises to meet the resistance, then remains stable as the muscle shortens”. In essence, this means that movement of a body part occurs, causing muscle fibers to shorten against some type of resistance, whether it be a weight, a resistance band, or gravity. Take the basic bicep curl, for example. When a person curls a weight upward, concentric contraction is occurring, as the biceps muscle fibers are shortening with the lifting of the arm. Conversely, eccentric muscle contraction occurs when a “muscle lengthens as the resistance is greater than the force the muscle is producing”. This means that movement of a body part is also occurring, but muscle fibers are lengthening against some type of resistance. Using the same bicep curl example from earlier, when a person lowers a weight from a curled position back downward, eccentric contraction is occurring, as the biceps muscle fibers are lengthening with the lowering of the arm.

(NOTE: There is a type of muscle contraction known as isotonic contraction, but that involves no movement of a body part. Think about trying to push against a brick wall – muscles are working, but your arms remain stationary, as the wall does not give way. However, our focus is on the two types of isotonic contractions – concentric and eccentric.)

Now that you hopefully have a better grasp on what concentric exercise and eccentric exercise are, you can understand that both types of contractions are used during a physical therapy program. Following hip or knee replacement surgery, exercises such as squats and steps often become staples. Examining the squat, the eccentric contraction occurs when a person lowers down from a standing position into the squatted position. This is due to the lengthening of the quadriceps and gluteal muscle fibers during this movement. The concentric contraction occurs when a person rises up to standing from a standing position due to the shortening of those same quadriceps and gluteal muscle fibers. Similarly, eccentric contraction occurs when a person steps down from a stair or stool, and concentric contraction occurs when a person steps up onto a stair or stool. Often, your physical therapist will have you focus on one movement or the other in order to isolate a specific type of muscle activity, depending on how you present clinically.

Now there is a rhyme and reason to these decisions. According to a meta-analysis performed by Roig et al. in 2009, “compared with concentric exercise, strength gains after eccentric exercise were more pronounced.” Roig et al. go on to report that eccentric exercise is more effective than concentric exercise in increasing muscle girth.” This means that eccentric specific exercises (steps downs and squat lowering, for example) result in more strength gain and muscle mass than concentric specific exercises (step ups and squat rising, for instance). It is suggested that eccentric exercises produce a higher demand on muscles, therefore requiring more muscle fiber recruitment and resulting in better gains in strength and muscle mass than concentric exercises. So, when your physical therapist asks you to perform those difficult squats or step downs, you now have a better understanding of the reasoning behind the decision.

My hope is that this article sheds some light on the kinematics behind the two types of isotonic strengthening exercises – that it, eccentric and concentric contraction. That way, you as the patient have a better understanding of why you do what you do during your therapy session. Following a joint replacement surgery, it is important to regain strength and muscle mass as quickly as possible so that you can stand, walk, go up and down stairs, and resume all desired activities. Eccentric exercises have been shown to cause desired changes more quickly, but your physical therapy program will still have a good mix of both eccentric exercise and concentric exercise. The bottom line is, keep moving and you’ll be back on your feet in no time!